1. Field of the Invention
The present invention relates to a control valve used for a hydraulic apparatus and, more particularly, to a control valve which can perform differential control and a four-wheel steering system including the control valve.
2. Description of the Related Art
A hydraulic apparatus which can perform differential control disclosed in FIG. 6 of France Patent No. 2,088,803 has been conventionally known.
According to this apparatus, a pressure chamber a pressure cylinder including a piston fixed to a steering rod of a steering unit of a vehicle communicates with a pressure chamber of a working cylinder including a piston coupled to a hydraulic control valve, and the working cylinder is operated by pressure generated in accordance with a change in volume which occurs in the pressure cylinder upon steering of the vehicle, thus controlling the opening degree of the valve. Since an orifice is formed in the piston of the working cylinder, an operation of the working cylinder is changed in accordance with a rate of change in pressure, i.e., a steering rate of a steering wheel.
In the above-mentioned conventional apparatus, however, the working cylinder is coupled to the hydraulic control valve through a wire to drive the valve. Therefore, in order to achieve an excellent operation, a mounting position and a mounting angle of the valve and the working cylinder are largely restricted, and a degree of freedom of mounting and design of the apparatus is undesirably insufficient. In addition, two pressure chambers of the pressure cylinder serving as a source for generating pilot pressure communicate with each other through the orifice. Therefore, when pressure output from another hydraulic control apparatus serving as a source for generating pilot pressure is used, the pressure output from another hydraulic control apparatus leaks from the orifice. In practice, in this conventional apparatus, pressure output from another hydraulic control apparatus cannot be used as pilot pressure, but an exclusive pilot pressure generating means must be provided.